The present invention relates to semiconductor devices, and more particularly, to semiconductor device packages comprising molded compounds.
Conventionally, many semiconductor devices are packaged by being encapsulated in molding compounds. A compound typically used in the industry comprises novalac epoxy.
More specifically, the typical conventional semiconductor device package comprises a supporting substrate on which a semiconductor die is mounted. The substrate may comprise a printed wire board, heatsink, leadframe, or the like, or integral combinations thereof. Electrical connections are typically made to the die via wire bonding. Conventionally, the supporting substrate, die and wire bonds are subsequently completely encapsulated with molding compound. Typically, leads extend from the compound for making electrical connections to the device. Alternately, conductive bumps may provide electrical connections to the semiconductor die.
The conventional packaging technique, including the complete encapsulation of the wires and semiconductor die in molding compound, gives rise to a number of significant disadvantages. For example, during the molding process, the molding compound contacts the wire bonded wires. The wires are subject to being pushed toward one another, causing shorts. Additionally, the wire bonds may be broken as the molding compound moves through the wires. To avoid this, conventional molding processes must go to great lengths to make the molding compound extremely viscous, thus minimizing disturbing the wires. Furthermore, special, extremely expensive, molding compounds are used to obtain the required viscosity.
An additional disadvantage occurring in conventional packaging processes is delamination of the molding compound from the semiconductor die. Ideally, the molding compound completely encapsulates the die, and remains in contact with the die throughout the life of the device. In practice, however, separation of the molding compound from the die may occur due to the difference in temperature characteristics such as coefficients of expansion, between the die and the compound. Conventionally, this problem is solved in part by employing extremely expensive molding compounds whose temperature characteristics are similar to the semiconductor die and other materials encapsulated. This problem is exaggerated as die size increases.
A related problem is that mold compound on the die surface can lead to stress related failure of the device. For example, the stress can cause metal movement on the surface of the die. Also, the stress at the compound/die interface and the difference in compound verses air dielectric properties can detrimentally effect electrical performance.
An additional disadvantage experienced with conventional packaging techniques is characterized as the popcorn phenomenon. During the molding process, small voids are formed deep within the device package. The voids can trap moisture. When the devices are incorporated into manufactured products, their temperature is typically raised substantially to flow solder. At the high temperatures, the moisture trapped in the voids turns to steam, thereby exploding the part. Conventional solutions to this problem include employing strict molding process parameters, expensive molding compounds, and extensive curing and dry packing the devices. Each of these solutions significantly raises the cost of the semiconductor device and has a negative impact on yield.
Further limitations of conventional packaging techniques include limitations on the number of semiconductor die which may be encapsulated to form a single packaged device, as well as the complexity of the die and wire layout within the molding compound.
Despite the discussed disadvantages, it is never the less extremely desirable to provide a semiconductor package which is essentially a molded package. This is because the molded package is rugged and fairly cost effective. Additionally, the industry is accustom to the molded package in that product designs and assembly processes are set up to use molded packages. Accordingly, what is needed is a device packaging technique which provides a molded package, yet eliminates the disadvantages brought about by conventional packaging.
Specifically, it would be desirable to provide a technique wherein the molding compound does not contact the wire bonded wires and semiconductor die during processing. This would eliminate the potential for wires shorting together or becoming detached. It would additionally be desirable if the molding compound was not in contact with the semiconductor die during the life of the device. This would eliminate the potential for separation between molding compound and semiconductor die. This would also eliminate the stress related problems resulting from compound covering the die. Furthermore, it would be desirable if molding compound was provided only near the perimeter of the complete device. This would significantly reduce the concern for voids and the popcorn condition.